Heretofore, Flon has been widely used as a refrigerant for a refrigerating machine and a heat pump (hereinafter referred to generically as "the refrigerating machine"). However, when discharged into the atmosphere, the Flon is accumulated and then decomposed by ultraviolet rays of the sun to produce chlorine atoms, and these chlorine atoms destroy the ozone layer having a function to protect the earth from the intensive ultraviolet rays of the sun. For this reason, the use of the Flon is getting limited. In recent years, much attention is thus paid to ammonia as an alternative refrigerant of the Flon.
An ammonia refrigerant does not destroy the environments of the earth in contrast to the Flon, and the refrigeration effect of ammonia is comparable to that of the Flon, and what is better, ammonia is inexpensive. However, ammonia is toxic, combustible, and insoluble in a mineral oil which is used as a lubricating oil for a compressor. In addition, ammonia has the drawback that its discharge temperature of the compressor is high. Accordingly, a refrigerating system which is now utilized is constituted so as not to bring about inconveniences owing to these drawbacks.
A typical constitution of the refrigerating system will be described in reference to FIG. 6. Reference numeral 50 is a direct expansion refrigerating system of a single-step compression type for providing heat of -10.degree. C. on the side of an evaporator and heat of +35.degree. C. on the side of a condenser. The function of this refrigerating system will be mainly described. An oil-containing ammonia refrigerant which is compressed by a refrigerant compressor 51 is treated in an oil separator 52 to separate the oil therefrom, and it is then subjected to heat exchange with a cooling water 64 in a condenser 53 (taken heat: about 35.degree. C.), whereby the ammonia refrigerant is condensed/liquefied in the condenser 53.
The oil liquefied and separated at the time of the condensation is further separated in an oil reservoir 55 disposed under the bottom of a high-pressure liquid receiver 54, and the ammonia refrigerant is then vaporized under reduced pressure through an expansion valve 56. In an evaporator 57, heat exchange is carried out with blast load fed by a fan 58 (taken heat: -10.degree. C.), and the ammonia refrigerant is then sucked into the compressor 51 via an ammonia oil separator 59. Afterward, this refrigerating cycle is repeated.
The oils stored on the bottoms of the oil separator 52, the oil reservoir 55 disposed at the bottom of the liquid receiver 54, the ammonia oil separator 59 and the evaporator 57 are all collected in an oil receiver 61 via oil drawing valves 60a, 60b, 60c and 60d, respectively, and the thus collected oil is returned to the compressor 51 through an oil jet portion 52a of the compressor 51 to carry out lubrication, sealing and cooling of sliding parts.
In this connection, it is well known that the refrigerating machine 50 can be applied as a heat pump device by taking out heat from the side of the condenser 53, and therefore, they will be generically called the refrigerating machine.
As the above-mentioned lubricating oil, there is usually used a mineral lubricating oil comprising of a paraffinic-based oil, a naphthenic-based oil or the like. However, since the lubricating oil is insoluble in ammonia, the oil separator is provided on the discharge side of the compressor to separate the ammonia gas and the lubricating oil discharged from the compressor. Even if the above-mentioned separator is provided, the lubricating oil in a mist state cannot be completely removed. Moreover, since the discharge side of the compressor has a high temperature, the lubricating oil is slightly dissolved in ammonia or the mist of the lubricating oil is mixed with ammonia, and the lubricating oil gets into the refrigerating cycle together with ammonia and tends to accumulate in pipe passages of the cycle because of being insoluble in ammonia and having a larger specific gravity than ammonia. Therefore, oil drawing portions 55, 60d are must be provided at the bottom of the high-pressure liquid receiver 54 and on the lower inlet side of the evaporator 57, respectively, and the oil separator 59 must be also provided on the gas suction side of the compressor 51. In addition, the separated oil, after recovered in the oil receiver 61, is required to return to the compressor again. In consequence, the constitution is noticeably complicate.
As described above, the lubrication oil is insoluble in the refrigerant, and therefore the oil tends to adhere to wall surfaces of heat exchange coils in the condenser 53 and the evaporator 57, so that a heat transfer efficiency deteriorates. Particularly in the evaporator having a low temperature, the viscosity of the oil increases and an oil drawing fluidity lowers, so that the heat transfer efficiency further deteriorates.
Therefore, it is necessary to separate the insoluble oil on the inlet side of the evaporator 57 as much as possible. However, if the refrigerant having a reduced pressure which has passed through the expansion valve 56 is introduced from the upper portion of the evaporator 57, the lubricating oil cannot be prevented from getting into the evaporator 57 owing to a difference between specific gravities, even if a specific separator is used. For this reason, the system having the above-mentioned constitution cannot help taking the so-called bottom feed structure in which the inlet portion of the refrigerant is disposed on the bottom of the evaporator 57.
However, if the bottom feed structure is taken, the so-called full liquid structure must be naturally taken in which the refrigerant can be discharged through the upper end of the evaporator against a gravity corresponding to the height of the evaporator 57, and as a result, a large amount of the refrigerant is required in the refrigerating cycle.
In the case of the above-mentioned ammonia refrigerating system, its use is limited to about -20.degree. C., but in recent years, the temperatures of industrial processes remarkably lower, and particularly in food fields, most of required refrigeration temperatures are -30.degree. C. or less from the viewpoints of preventing the melting of fat at the time of thawing and keeping qualities. Particularly in the case of an expensive food such as tuna, a freezing preservation temperature is very low, in the range of -50.degree. C. to -60.degree. C.
Such a freezing temperature cannot be obtained by the above-mentioned single-step compressor, and in general, a two-step compressor is used. However, when the temperature of the evaporator is cooled to -40.degree. C. or less by means of the above-mentioned conventional technique, the fluidity of the lubricating oil noticeably lowers as shown in Table 3 given below, so that the evaporator is liable to be clogged.
In order to overcome the above-mentioned drawback, such an extremely low temperature ammonia two-step compression type liquid pump recycling system as shown in FIG. 7 has been suggested.
The constitution of the suggested recycling system will be briefly described mainly in reference to differences between this recycling system and the above-mentioned conventional technique. A compressed liquid discharged from the high-pressure liquid receiver 54 to a liquid pipe 66 cools the interior of an intermediate cooler 68 by an expansion valve 67. On the other hand, the terminal end of the liquid pipe 66 is introduced into a supercooling pipe 69 in the intermediate cooler 68, and the compressed liquid is then cooled to about -10.degree. C. in the subcooling pipe 69. Afterward, the compressed liquid is vaporized under reduced pressure by an expansion valve 74 to be introduced into a low-pressure liquid receiver 70.
As a result, the refrigerant cooled to from -40.degree. to -50.degree. C. or less is stored in the liquid receiver 70.
This refrigerant is introduced into an evaporator 73 via a liquid pump 71 and a flow rate regulating valve 72, and the refrigerant evaporated by heat exchange (taken heat: -40.degree. C.) with blast load fed by a fan 74 in the evaporator 73 is introduced into the low-pressure liquid receiver 70 to be cooled and condensed/liquefied.
On the other hand, the evaporated refrigerant in the low-pressure liquid receiver 70 is sucked into a low step compressor 75 and compressed, and this compressed gas is cooled in the intermediate cooler 68 and then introduced into the supercooling pipe 69 for heat exchange in the intermediate cooler 68 to supercool the condensed refrigerant coming through the above-mentioned liquid pipe 66 to about -10.degree. C. The thus supercooled liquid is vaporized under reduced pressure by the expansion valve 74, while introduced into the low-pressure liquid receiver 70.
The vaporized refrigerant in the intermediate cooler 68 is compressed by a high step compressor 51', and this cycle is then repeated.
Under all of the high-pressure liquid receiver 54, the intermediate cooler 68 and the low-pressure liquid receiver 70, the oil reservoirs 55, 68a and 70a are disposed, respectively, and the separated oils in these reservoirs are collected in the oil receiver 61 and then returned again to oil jet portions 51a, 75a on the sides of compressor 51' and 75. In this connection, reference numeral 76 in the drawing is a liquid surface float valve.
However, also in such a conventional technique, fundamental drawbacks such as the complication of the oil recovery constitution and the deterioration of the heat transfer efficiency cannot be overcome. Particularly on the side of the above-mentioned low-pressure liquid receiver 70, the refrigerant cooled to from -40.degree. to -50.degree. C. is stored, so that the lubricating oil stored in its oil reservoir is similarly cooled to from about -40.degree. to -50.degree. C., so that the fluidity of the lubricating oil noticeably deteriorates. Thus, when the oil is drawn, it is necessary to temporarily raise the temperature of the oil, and as a result, the continuous operation of the refrigeration cycle is disturbed. In consequence, the maintenance that the above-mentioned cycle is stopped to recover the oil is necessary, each time the oil is accumulated as much as a predetermined amount.
On the other hand, an enclosed compressor is often used in a domestic refrigerator or air conditioner, and CFC and HCFC refrigerants such as dichlorodifluoromethane (R12) and chlorodifluoromethane (R22) have been heretofore used. In the future, HFC containing no chlorine, for example, 1,1,1,2-tetrafluoroethane (R134a) will be used, but such a Flon is expensive. On the other hand, ammonia is more inexpensive than the above-mentioned Flons. In addition, ammonia is excellent in the heat transfer efficiency, has a high allowable temperature (a critical temperature) and a high allowable pressure as the refrigerant, is soluble in water to prevent the expansion valve from plugging, and has large evaporation latent heat to exert a large refrigeration effect. For these reasons, the employment of ammonia is advantageous. However, the enclosed compressor has a structure in which an electric motor and the compressor are integrally enclosed, and therefore ammonia itself corrodes copper-based materials, which makes the use of ammonia impossible. In addition, since ammonia is insoluble with the lubricating oil, it is extremely difficult to recover and recycle the oil alone. For these reasons, ammonia cannot be used nowadays.
However, if a lubricating oil which has an excellent solubility with ammonia and in which quality does not deteriorate even by a long-term use is developed, most of the above-mentioned problems will be solved.
The lubricating oil having such a solubility has already been suggested in the field of the Flon, and for example, an ester of a polyvalent alcohol and a polyoxyalkylene glycol series compound are known. However, any example of the lubricating oil for the ammonia refrigerant has not been present. Ammonia is strongly reactive, and so even when the ester slightly hydrolyzes, an acid amide is formed which causes a sludge to deposit. Moreover, these kinds of lubricating oils are poor in the solubility with ammonia, and hence it is difficult to use these lubricating oils in combination with the ammonia refrigerant.
In view of such technical problems, an object of the present invention is to provide a working fluid composition for a refrigerating machine (hereinafter referred to simply as "the working fluid composition") which is extremely excellent in the solubility with the ammonia refrigerant and which can be obtained by mixing a lubricating oil having excellent lubricating properties and stability with an ammonia refrigerant.
Another object of the present invention is to provide a refrigerating machine suitable for the above-mentioned working fluid composition.
Still another object of the present invention is to provide a method for lubricating a refrigerating machine and a refrigerating compressor mounted in the refrigerating machine by the use of the above-mentioned working fluid composition, and according to this method, the above-mentioned drawbacks of ammonia can be removed.